Fluorescent tracer processes employing a fluorescent europium chelate

ABSTRACT

An inspection process and inspection tracer compositions which utilize fluorescent coordination compounds dissolved in liquid or resinous carriers. The fluorescent metal-organic coordination compounds provide unique effects of transparency to white light along with novel characteristics of fluorescent color. The tracer compositions disclosed may be utilized for the detection of surface defects in parts, as display elements (image-forming screens), or as marking materials.

United States Patent Inventor lama R. Albnrger 5007 Hillard Ave, LaCanada, Calif. 91011 Appl. No. 711,105

Filed Mar. 6,1968

Patented Mar. 2, 1971 FLUORESCENT TRACER PROCESSES EMIPLUYIING AFLUORESCENT EUROIPIUM CHIElLA'liE 2 Claims, 3 Drawing Figs.

11.8. C1 250/71, 73/104, 252/301 .2R, 260/429J lint. Cl G01n 21/16, G0ln21/38; C09k 1/00 lField o1 Search 252/301.2,

[56] References Cited UNITED STATES PATENTS 3,214,382 10/1965 Windsor252/30l.2 3,225,307 12/1965 Weissman 331/945 3,367,910 2/1968 Newing260/465 3,377,292 4/1968 Halverson 252/30l.3 3,388,071 6/1968 Nehrich eta1.. 252/301.2 3,398,099 8/1968 Kleinerman 252/301.2 3,440,173 4/1969Hovey et a1 252/30l.2 3,450,641 6/ 1969 Schimitscher et a1 252/301.23,360,478 12/ 1967 Schimitscher et al.. 252/301.2 2,871,697 3/1959Sockman 73/104 3,028,338 4/1962 Parker 252/301.2 OTHER REFERENCES COLLATet a]. Anal. Chem, 27(1955), p.961- 5 Primary Examiner-Tobias E. LevowAssistant ExaminerA. P. Demers ABSTRACT: An inspection process andinspection tracer compositions which utilize fluorescent coordinationcompounds dissolved in liquid or resinous carriers. The fluorescentmetal-organic coordination compounds provide unique effects oftransparency to white light along with novel characteristics offluorescent color. The tracer compositions disclosed may be utilized forthe detection of surface defects in parts, as display elements(image-forming screens), or as marking materials.

FLUORESCENT TRACER IIROCESSES EMELOYEWI G A FLUORESCENT EIJEOlPlI UIl/lCHELATE The present application is a continuation-in-part of mycopending application, Ser. No. 492,674, filed Oct. 4, 1965, now Pat.No. 3,386,920 issued Jun. 4, 1968, for PROCESS FLUORESCENCE DETECTION OFEXTREMELY SMALL ELAWS, issued as US. Pat. No. 3,386,920, now Reissue No.Re-26,888, which latter application was a continuation-in-part of myapplication, Ser. No. 323,529, filed Nov. 13, 1963, for FLUORESCENTTRACERS, which other application was a continuation-in-part of myapplication Ser. No. 149,061, filed Oct. 31, 1961, for FLUORESCENTTRACERS, which last application was a continuation-in-part of myapplication, Ser. No. 82,374, filed Ian. 13, 1961, for FLUORESCENTPENETRANT TRACERS. The present invention relates to fluorescent tracerswhich exhibit fluorescence response in relatively thin films, and, moreparticularly, to a method of preparing ultraviolet-responsivefluorescent tracers utilizing fluorescent metal-organic coordinationcompounds as ultraviolet-responsive ingredients.

Fluorescent tracers are well-known in the prior art, and have usuallybeen comprised of one or more fluorescent dyes dissolved in a suitablecarrier material. These tracers have found advantageous usage inindustrial inspection processes. Thus, such tracers have been employedin the detection of surface flaws in parts constructed of metal,ceramic, or other material. When used for this purpose, the fluorescentdye and carrier is utilized in the form of a penetrant liquid whichforms entrapments in the flaws and renders the latter more readilydetectable than might be the case with ordinary visible colored dyes.

Fluorescent tracers are also employed in the form of solutions offluorescent indicator dyes in thin layers of plastic or resinousmaterials, and such materials may be used in the construction ofultraviolet-responsive image-forming screens, as set forth in mynow-issued Pat. No. 3,320,417, granted May 16, 1967, for FLUORESCENTIMAGE-FORMING SCREEN, or as controlled concentrations of fluorescentdyes in liquid, plastic, or resinous materials, for purposes ofadjusting and controlling the thickness of applied films of material inaccordance with tee method of my now-issued Pat. No. 3,341,706, grantedSept. 12, 1967, for METHOD OF CON- TROLLING THE THICKNESS OF APPLIEDLIQUID FILMS USING DYE TRACERS.

In my copending application, Ser. No. 492,674, of which this applicationis a continuation-in-part, l have pointed out that in the use offluorescent penetrants for detection of exrnely small flaws, itheretofore been thought that the at lity of the tracer to detect theflaws is a function of its fluorescent brightness. Basically,fluorescent brightness, of course, depends upon the particularfluorescent dye or dyes used. However, such bri. itness can be enhancedby the well lrnown effect of cascading of fluorescence. Whereas thebrightness effecw produced by combinations of two or more fluorescentdyes are ordinarily approximately additive, cascading involves thetransfer of radiant energy from one dye component to another, withconsequent increase in brightness merely additive.

Since, as has been pointed out, the sensitivity of a fluorescentsubstance as regards its ability to reveal its presence in microtraceshas been equated with fluorescent brightness, it has often been thepractice in industrial inspection processes to attempt to maximize suchsensitivity of the fluorescent tracer used by increasing the fluorescentbrightness thereof; e. g., through the use of the aforementioned effectof cascading of fluorescence. Thus, when a flaw and its tracerentrapment are so small as to be virtually invisible under black light,it has usually been felt necessary to employ a tracer of increasedfluorescent brightness so as to make the flaw detectable. However, inspite of the attempts heretofore made to maximize the sensitivity offluorescent penetrant materials by increasing the fluorescent brightnessthereof, presently known fluorescent penetrant tracers are unable todetect extremely small flaws which may result from such effects asintercrystalline corrosion or creep cracks, and which may havedimensional magnitudes on the order of lO. to 10. centimeters.

Failure in the prior art to produce fluorescent tracers which are ableto detect flaws of such small dimensional magnitude has resultedprimarily from the emphasis which has been laid on the fluorescentbrightness of the particular dyes employed, as discussed above.Furthermore, it should be noted tat in the design of fluorescent tracerliquid compositions in the past, attention has been concentrated onusage of fluorescent dyes or dye-type materials, without anyconsideration being given to possible advantages to be gained throughuse of nondye materials, such as fluorescent metal-organic coordinationcompounds.

I have found that in certain kinds of fluorescent materials, known ascoordination compounds, many of such materials do not appear to sufferfrom the effects of self-quenching of fluorescence in highconcentrations. In addition, I have found that certain of such materialsexhibit unique and desirable properties of color, transparency,stability, and dimensional sensitivity, as well as low cost, when theyare utilized in appropriate solvent carriers, or in resinous carriers asimageforming screens, or as finely ground powders for usage asfluorescent pigments.

The principal object of the invention, therefore, is to provide improvedfluorescent tracer materials, utilizing fluorescent coordinationcompounds.

Another object of the invention is to provide fluorescent tracerprocesses employing fluorescent coordination compounds.

Still another object of the invention is to provide a method ofpreparing fluorescent tracer materials having improved transparency andcolor characteristics.

A further object of the invention is to provide improved fluorescentfeatures for use in fluorescent image-forming screens.

A still further object of the invention is to provide improvedfluorescent pigment materials.

These and other objects of the invention will in part be obvious andwill in part become apparent from the following description thereof whenread in conjunction with the accompanying drawings, in which:

FIG. I is a graph illustrating typical fluorescence response transitioneffects of fluorescent coordination compounds, as used in the method ofthe invention.

FIG. 2 is a chart employable in conjunction with the use of thefluorescent tracers of the invention, a reading of said chart for aparticular tracer being shown; and

FIG. 3 is a diagrammatic representation of a thin film surface coatingillustrating the use of tracer materials of the invention.

The present invention provides fluorescent tracers, each of which isformulated essentially by the solution in a suitable solvent of afluorescent metal-organic coordination compound selected from a group tobe defined hereinbelow, and at least to a minimum level ofconcentration, up to a maximum level of concentration, depending on thesolubility limit of the coordination compound, as will be described. Inorder to understand the reason for the set minimum level ofconcentration, reference should first be made to the concept, which Ihave formulated, of thin-film fluorescencez In accordance with thisconcept, a fluorescent sensitizer, such as a fluorescent dye or othersubstance, as, for example, a fluorescent coordination compound in thepresent instance, when in solution exhibits the characteristic of athreshold of film thickness below which fluorescent coordinationcompound dissolved in a particular carrier at a particularconcentration, there exists a specific film thickness below which thereis essentially no fluorescence, and above which there is a fluorescenceresponse. The threshold thickness of the tracer film may be termed thedimensional sensitivity" of the fluorescent tracer. The concept ofthin-film fluorescence is applicable to a fluorescent substance insolution in a solid form, (e.g., as a plastic or resinous material) aswell as in liquid form.

For the purpose of measuring thedimensional sensitivity of a fluorescentmaterial in solution, and also of determining the applicable physicalconstants thereof, I employ a Meniscus- Method, which is described in myU.S. Pat. No. 3,107,298, granted Oct. 15, 1963, for APPARATUS FOR THEMEA- SUREMENT OF FLUORESCENT TRACER SENSITIVITY. In practicing theinvention disclosed in the latter patent, a flat black glass platen ispositioned under a black light, and a drop of liquid having dissolvedtherein, a fluorescent substance is placed on the platen. A convex clearglass lens having a preferred radius of curvature of 106 cm. is thenplaced over the drop of liquid, and so as to rest on the platen. At thepoint of contact between the lens and the platen, the liquid film has athickness of substantially zero, and a meniscus-shaped film surroundssaid contact point.

As pointed out in Pat. No. 3,107,298, the thickness of the liquid filmvaries continuously with the radial distance from the contact point.However, a fluorescence response as seen under black light is distinctlya nonuniform function, so that a nonfluorescent spot is seen in theregion of the contact point. When the flat platen is made of blackglass, the nonfluorescent spot appears as a black spot which contrastssharply with the so surrounding area of fluorescence, and which can bemeasured as to its diameter with good accuracy. The diameter of thisblack spot is used as a measure of the film thickness above whichfluorescence response occurs, and below which fluorescence essentiallyceases. For a given fluorescent substance dissolved in a particularsolvent material, the diameter of the black spot varies depending uponthe concentration of the fluorescent substance.

Inasmuch as the transition of fluorescence response, with respect tofilm thickness, is really a continuous function, the toe of thetransition curve approaches zero response as the film thickness if madesmaller, but, theoretically at least, never actually reaches zero. As apractical matter, in locating the point corresponding to the so-calleddimensional threshold of fluorescence, a point on the transitioncharacteristic curve of the tracer is taken at a film thickness which isone-tenth the film thickness at the midpoint of the transition curve,where the brightness of fluorescence response falls to a value belowabout 2 percent to 7 percent of the maximum brightness which appears ina relatively thick film of the tracer composition. In theabove-mentioned Meniscus-Method test, the diameter of the black spotwould be taken as the distance between the points where brightnessresponse begins to rise fairly steeply, at the 2 percent to 7 percentvalues of maximum brightness brightness, as indicated above. In mycopending application, Ser. No. 492,674, I have described in detailprocedures for assigning ratings of dimensional sensitivity to solutionsof fluorescent substances. Also, I have described methods which areappropriate. for assigning ratings of Specific Sensitivity to thefluorescent substances themselves.

Referring now to FIG. I, there is shown here a graph which illustratesthe effect of fluorescence response transition which may be found in afluorescent sensitizer dye or a fluorescent metal-organic coordinationcompound, as the case may be. In this graph, the axis of abscissas isset forth on a logarithmic scale in terms of film thickness, and theaxis of ordinates is set forth on a linear scale in terms of relativebrightness from zero to unity.

When a fluorescent substance is s dissolved in a solvent carrier to agiven concentration, and the transition effect is measured by theabove-described Meniscus-Method, a fluorescence transition curve isobtained similar to curve R0. The midpoint of this transition curve isshown at point 11, at a value of relative brightness of .5, and thepoint of dimensional threshold is at point 112, which is at a filmthickness one decimal order of magnitude smaller than that at point 1 1.

When the concentration of the dissolved fluorescent substance isincreased, a new transition curve is obtained similar to curve 13, thisnew curve having essentially the same shape as curve 10, except that itis offset to the left along the axis of abscissas. The dimensionalthreshold of fluorescence for the new transition curve 13 is now foundat point 14. The points of fluorescence threshold 12 and 14 usually fallin the dimensional range from about 20,000 millimicrons down to about 40millimicrons or less; that is, for useful tracer materials.

In order for fluorescent substance to be useful as a dyetracer, it mustfirst be soluble in a suitable solvent carrier. Second, it must becapable of yielding a dimensional sensitivity value smaller than about20,000 millimicrons. This latter performance requirement for afluorescent tracer material is, of course, arbitrary, however, practicalconsiderations of tracer performance for the detection of surface flawsby means of inspection penetrants, for example, or for fluorescencedetection of thin coatings of tracer-tagged materials require that thedimensional threshold of fluorescence response must be smaller thanabout 20,000 millimicrons; that is, if the tracer material is to beuseful.

In cases where the fluorescent tracer composition is to be used as afluorescent image-forming screen or a transfer wax for antifraudpurposes, it may be necessary to provide a dimensional thresholdcondition in the tracer composition which is as small or smaller thanabout 1000 millimicrons, or even millimicrons.

If the values of dimensional threshold for a fluorescent substance areplotted with respect to various values of dye concentration, then aso-called dilution curve is obtained as shown in FIG. 2. Referring nowto FIG. 2, there is here shown a graph in which the axis of abscissas isset forth on a logarithmic scale in terms of concentration of afluorescent coordination compound, and in which the axis of ordinates isset forth on a logarithmic scale in terms of film thickness orMeniscus-Method spot diameter or radial distance from spot center, asmay be desired. The diagonal line 20 is a locus line for dimensionalthreshold values for a typical fluorescent coordination compound of theinvention, a point 21 on this locus line being determined by theintersection of lines 22 and 23, which represent measured values ofconcentration and film thickness, respectively.

It will be seen from an examination of FIG. 2 that if line 20 representsthe locus of dimensional threshold values for a typical fluorescentcoordination compound, then in order for the coordination compoundsubstance to provide a useful effect as a tracer indicator, it must bepresent in solution at a concentration which is greater than about .1gram per pint. The unit of concentration measure grams per pint is usedhere for the reason that values in grams per pint are numerically equalto pounds per 55 gallon quantity, and thus a convenient translation ispermitted from laboratory tests to production quantities of material.

Measurements of dilution characteristics for various of the coordinationcompounds which are useful in the method of this invention may yielddilution curves corresponding to curves 20, 24, or 25, or other curvesin similar locations on the chart. In any event, for a material to beuseful as an inspection tracer, the concentration of the coordinationcompound must be greater than about .1 gram per pint, and thedimensional threshold condition thus provided must be smaller than about20,000 millimicrons. The term inspection tracer refers, as indicatedabove, to compositions containing a dissolved fluorescent ingredient andwhich yields a fluorescent response in film thicknesses smaller than20,000 millimicrons.

Referring now to FIG. 3, a surface 30 of a substrate 31 is coated with athin film of plastic material 32 which is to act as an electricalinsulating layer. For the purpose of this illustration, it is desiredteat the thickness of the applied insulating layer shall be smaller thanthe wavelength of light or 500 millimicrons. At the same time, it may bedesired that the presence of this thin insulating layer will be revealedby its fluorescence response. It will be seen, therefore, that if acoordination compound used as a tracer indicator in the plastic materialhas a dilution characteristic corresponding to curve 2 h of 2, then inorder for such a layer to yield a fluorescence response, theconcentration of the coordination compound must be greater than about 20grams per pint.

Again referring to Fit}. 3, the layer 32 may be formed by laminating amelted mass of plastic material between two layers of glass, afluorescent coordination compound being dissolved in the plastic, or thefluorescent coordination compound may be impregnated into the surface ofa plastic substrate, thus forming the layer 32. Structures of thisnature may be used as fluorescent image-forming screens, as will bedescribed in Example 1 below.

E have discovered that certain fluorescent metal-organic coordinationcompounds exhibit unique and useful features with respect to inspectiontracer behavior. First, though, it should be mentioned that fluorescentcoordination compounds have been employed in the past mostly for thepurposes of chemical analysis and identification of metals. in such useapplications, a given fluorescent coordination compound is formed by achemical reaction of a metal ion with a socalled ligand," and theresulting coordination compound is precipitated out of solution, ormicroscopic quantities of the resultant compound are maintained indispersion in the liquid, being revealed by their fluorescence responseunder black light excitation. The extraordinary stability of suchprecipitates or dispersed compounds serves to yield an extremely highlevel of sensitivity in the detection of minute quantities of certainmetal elements. Metal-organic coordination compounds are sometimes knownas metal-organic chelates, or metal-organic complexes.

A number of different metal elements may be used in preparingmetal-organic coordination compounds, or complexes, or chelates. Amongsuch metals are: aluminum, barium, beryllium, cadmium, calcium, cerium,dysprosium, erbium, europium, gadolinium, gallium, gold, indium,lutecium, magnesium, niobium, ruthenium, samarium, scandium, strontiurn,terbium, thorium, vanadium, ytterbium, yttrium, zinc, and zirconium. Thefluorescent coordination compounds, as used in the method of theinvention, are formed by reaction of a metal ion with an organic ligand,the resulting coordination compound being purified by an appropriatesolvent extraction or precipitation, as required.

Among the various useful organic ligands which may be employed are:fluorescent-chelate-forming flavonals, 3-hydroxyllavone (flavonol), 2'3,4'5, 7-pentahydroxyflavone (Morin), 3,3,4,5,7- pentahydroxyflavone(Quercetin), fluorescentchelate-forming oxyquinolines,li-hydroxyquinoline, Z-methylfi-hydroxyquinoline (8-hydroxyquinaldine),5-sulfo-8-hydroxyquinoline 5,7-dichloro-8-hydroxyquinoline,fluorescent-chelateforming anthraquinones,l-aminol-hydroxyanthraquinone, fluorescent-chelate-formingsalicylidenes, salicylidene-or aminophenol,N-salicylidene-2amino-3-hydroxyfluorene,salicylidene-o-aminophenoll-benzol, fluorescent-chelate-formingazobenzenes, .7.,2, l'thrihydroxyazobenzene, 2,2 dihydroxylAl-dichloro-asobenzene, 2,2 -dihydroxyazobenzene, 2, 2' -dihydroxy-l.l-naphthaleneazobenzenefi-sodium sulfonate, fluorescent-chelate-formingsalicylal dehydes, fi-chloro-ll-hydroxybenzaldehyde, salicylaldehyde(o-hydroxybenzaldehyde), fluorescent-chelate-forming betadiltetones,ll-phenyl-l-l, 2-propanedione, l-pheyl-ll,3-butanedione (benzoylacetone), l,3-diphenyl-i,El-propanedione (dibenzoyl methane),lAA-trifluoro-Zl-phenyl-l,3-butanedione, l,dAl-trifluoro-l-(Z-napth(2-naphthyl)-l,3-butanedione, d,l, l-trifluoro-l-(2thienyl)-ll,3-butanedione, 6,4,4-trifluoroll-(Z-furyl-l, S-butanedione,2,2 -bipyridine, l,ltl-phenanthroline, 3-hydroxy-2-naphthoic acid,5-hydroxy' fi methyl-lAl-pyron (.lfiojic acid), and benzoin.

in the above listings of metal elements and ligand materials, it will beunderstood that not all ligands may be used with each and every one ofthe listed metals to produce a useful fluorescent coordination compound.However, each ligand listed will form a useful fluorescent coordinationcompound with at least one of the listed metal elements, and in somecases, a number of the metal elements may be reacted with a givenligand, or a number of different ligands may be reacted with a givenmetal. For example, aluminum may be reacted with a2',3,4,5,7-pentahydroxyflavone, 2,2'-dihydroxy-ll,ll naphthaleneazobenzene-S- sodium sulfonate,N-salicylidenelaminoQ-hydroxyfluorene, ll-hydroxyquinoline, and 3-hydroxy-Z-naphthoic acid. Also, S-hydroxyquinoline may be reacted withzinc, calcium, cadmium, magnesium, and various other metals as well asaluminum, to produce fluorescent coordination compounds. Some of themore useful coordination compounds which exhibit especially uniqueproperties when employed in the method of the invention are:betadiketone complexes with europium, such as Tris-(4AA-trifluoro-l-(Z-thienyl)-l,3-butanedione)-europium, and various of themetal celates with fl-hydroxyquinoline. Materials such as these aresubstantially colorless in ordinary white light and may be used to formbrightly fluorescent ultravioletresponsive transparent image-formingscreens.

In many instances in the past, metal-organic coordination compounds havebeen utilized for their features of insolubility, e.g., their ability toform precipitates. Certain types of coordination compounds, notably thevarious metal complexes with g-hydroxyquinoline, are sometimes employedas fluorescent pigments, and are not normally considered to be solublesubstances. 1 have discovered that such materials can be made to sdissolve to relatively high concentrations in certain properly selectedsolvents. The oxyquinoline complexes, for example, may be dissolved inhot dimethyl formamide or N-methyl pyrrolidone, and they may then bedispersed in acrylic or styrene resins, or in some cases, they may bedissolved directly in a hot plastic melt.

Certain groups of the coordination compounds useful in the method ofthis invention are soluble in ketones, such as acetone, methyl ethylketone, methyl isobutyl ketone, and diacetone alcohol, methyl alcohols,such as methanol, ethanol, isopropanol, butanol, and isodecanol,glycols, such as diethylene glycol, polyethylene glycol, and glycerin,glycol others, such as ethylene glycol monoethyl ether, and ethyleneglycol monobutyl ether, plastic materials, such as polymerized methylmethacrylate, butyl methacrylate, para-toluene-sulfonamide, celluloseacetate, vinyl plastics, polystyrene, and liquid epoxy resins, waxymaterials, such as triphenyl phosphate, carnauba wax, beeswax, andparaffin, and miscellaneous solvent materials, such as nitroethane,dimethyl sulfoxide, ethyl ether, dioctyl phthalate, N-methylpyrrolidone, methyl cellosolve acetate, tetrahydrofuran, silicone oils,and chlorinated hydrocarbons, such as methylene chloride,trichloroethylene, and perchloroethylene. Many of the materials may alsobe dissolved in water-insoluble inspection penetrants, water-solubleinspection penetrants, and wateremulsitiable inspection penetrants.

EXAMPLE I A solution of a coordination compound was prepared as follows,so as to provide a concentration of about 5 grams per pint:

l,3-butanediono)-europium dimethyl formamide 20 ml.

The above solution was be colorless in ordinary light, while exhibitingan intense red fluorescence under ultraviolet light. A drop of thesolution was measured by use of the Meniscus Method instrumentation, andit was found that its dimensional threshold of fluorescence was about900 millimicrons. About 25 grams of polybutyl methacrylate were added tothe above composition, and the mixture was heated to dissolve theplastic substance. Heating was continued until all of the dimethylformamide was driven off by evaporation, leaving a transparent mixturehaving a faint amber coloration. This plastic melt was allowed to standat a temperature just above its melting point so as to remove allbubbles, after which a few drops of the plastic melt were placed on aclear glass plate .211 gram which was warmed on a hot plate to keep theplastic in a fluid condition. A second glass plate was placed over thepuddle of fluid plastic mixture, and the two glass plates were gentlypressed together so as to form a laminated layer. A water clear andcolorless layer of acrylic plastic containing a dissolved fluorescentcoordination compound was thus produced. It was found that this clearplate could be used-as a an ultravioletresponsive fluorescentimage-forming screen, such that ultraviolet images projected onto theplate would yield intensely fluorescent red images.

The hot melt of resin was extended by the addition of about 450 grams ofpolybutyl methacrylate to provide a concentration of the europiumcoordination compound of about .1 gram per pint volume of material, anda dimensional threshold of fluorescence in the range of 20,000millimicrons. The resulting mixture was cooled to a solid, crushed to agranular consistency, and was then pulverized to a fine powder in amicropulverizer hammer mill. The resulting low-cost powder was found tobe an excellent pigment suitable for use in formulating fluorescent redchalks, paints, and dusting powders.

EXAMPLE ll A solution of a coordination compound was prepared asfollows:

bis-(S-quinolinolate)-zinc 6 grams Epoxy Resin (Epon 815, Shell ChemicalCo.) 200 grams The above formulation was heated to a temperature ofabout 275 F., at which temperature the mixture cleared to a translucentsolution. A specimen of the fluorescent liquid mixture was examinedusing the Meniscus-Method instrument, and it was found that thedimensional threshold of fluorescence was in the range of about 1000millimicrons or less.

The solution was cooled to room temperature, and small test quantitieswere used as follows to prepare transparent fluorescent image-formingscreens. A test quantity of the above fluorescent epoxy resin, in theamount of about 10 milliliters, was placed in a small cup and about 1.25ml. of diethylenetriamine catalyst was added, the mixture being stirredthoroughly. A few drops of the catalyzed mixture were placed on a clearglass plate, and a cover glass was placed over the puddle of liquid, thetwo plates being gently pressed together to form a fluorescent laminatelayer about .001 inch thick.

The thus-fabricated laminated plate was allowed to cure bypolymerization, and the sr resulting structure was found to bepractically water-clear and colorless in white light, yet underexcitation by projected ultraviolet images it yielded a bright greenishyellow fluorescence.

EXAMPLE III EXAMPLE IV A series of stamping inks similar to that ofExample III were prepared by dissolving the calcium, magnesium,aluminum, and zinc coordination compounds with the -hydroxyquinolineligand in hot dimethyl formamide. in each case, the mixture ofcoordination compound and solvent was cooled to room temperature andfiltered to remove undissolved material, leaving a saturated solution ofthe fluorescent ingredient. The resulting fluorescent liquid was thenused to saturate a porous rubber stamp pad element. The markingimpressions 1 gram from the thus-prepared inks were colorless in whitelight and appeared brightly fluorescent under black light, providingfluorescent colors ranging from bluish green to greenish yellow.

EXAMPLE V A fluorescent liquid having the following formulation wasprepared:

tris-(4,4l,4-trifluoro-l-(2-thienyl) The above liquid formulation wasused as a bath for impregnating the fluorescent ingredient into sheetsof clear vinyl plastic by a procedure similar to that disclosed in myabovementioned US. Pat. No. 3,320,417. A sheet of clear vinyl plasticwas dipped into the above fluorescent liquid for about 5 minutes at roomtemperature. The plastic sheet was removed from the bath and was drainedand dried. it was found to be water-clear and colorless in ordinarywhite light, while under black light excitation, it exhibited an intensered fluorescence. The thus-prepared plastic sheet was suitable for useas a transparent ultraviolet-responsive image-forming screen.

The above-described fluorescent liquid was put in a liquid cellconsisting of two glass plates bound together such that they wereparallel and spaced about .05 inch apart, and with the edges sealed soas to contain the liquid. This cell and liquid assembly was found toperform as a transparent window. Ultraviolet images projected onto theliquid layer in this window produced an intense red fluorescenceresponse.

EXAMPLE VI A fluorescent transfer wax was prepared as follows:

triphenyl phosphate grams carnauba wax 400 gramstris-(4,4,4-trifluoro-1-(2-thienyl) -1,3-butanediono)-europium 5 gramsThe a wax and triphenyl phosphate ingredients were melted in a beaker,and the europium complex was added and dissolved in the melt. The meltedmixture was coated onto a paper substrate by a blade-coating method, andit was found that the resulting coating provided a satisfactory transfermark with the impression of a pencil or a ballpoint pen. A larger batchof the same mixture was prepared and used on a heated printing press ina spot-printing application on signature transfer slips for use by banksfor antifraud purposes. The transferred signatures from the signatureslips were completely colorless and invisible in ordinary white light,while under black light, they showed as a bright red fluorescence.

Although the invention has been described with reference to particularembodiments thereof, it will be understood that various changes may bemade therein, particularly from the standpoint of minor structuralmodifications of ligand materials, without departing from the spirit ofthe invention or the scope of the appended claims.

I claim:

1. in an inspection process in which thin films of a fluorescent tracerare revealed by a fluorescence response and wherein a fluorescent traceris applied to a test surface and said test surface is inspected underblack light for the presence or absence of said fluorescent tracer, theimprovement being the step of applying onto a test surface a fluorescenttracer comprising the metal-organic coordination compound tris-4,4,d-trifluoro-l-(2-thienyl)-butanediono-europium dissolved in asolvent carrier, said coordination compound being present in saidsolvent carrier within the range of proportional concentrations from atleast about .1 gram per pint up to the limit of solubility of saidcoordination compound, sufficient to provide a dimensional threshold offluorescence with an operational value below about 20,000 millimicrons.

acetate, vinyl plastics, triphenyl phosophate, camauba wax, beeswax,paraffin, nitroethane, dime'tliyl sulfoxide, ethyl ether, dioctylphthalate, N-methyl pyrrolidone, methyl cellosolve acetate,tetrahydrofuran, silicone oils, methylene chloride, trichloroethylene,perchloroethylene, water-insoluble inspection penetrants, water-solubleinspection penetrants, and water-emulsifiable inspection penetrants.

2. An inspection process in accordance with claim 1 in which saidsolvent carrier is at least one member selected from the groupconsisting of acetone, methylethyl ketone, methyl isobutyl ketone,diacetone alcohol, methanol, ethanol, isopropanol, butanol, isodecanol,diethylene glycol, polyethylene glycol, glycerin, ethylene glycolmonoethyl ether, ethylene glycol monobutyl ether, polymerized methylmethacrylate, polymerized butyl methacrylate, para-toluene sulfonamide,polystyrene, liquid epoxy resins, cellulose acetate, vinyl plastics,triphenyl phosophate, carnauba wax, beeswax, paraffin, nitroethane,dimethyl sulfoxide, ethyl ether, dioctyl phthalate, N-methylpyrrolidone, methyl cellosolve acetate, tetrahydrofuran, silicone oils,methylene chloride, trichloroethylene, perchloroethylene,water-insoluble inspection penetrants, water-soluble inspectionpenetrants, and water-emulsifiable inspection penetrants.